5. COMMENTS ON STONE CONSOLIDANTS

Although stone consolidants have been extensively used for over a century, their selection is still largely based on empirical considerations. If a consolidant appears to give acceptable results with one type of stone, it is often applied to other types of stone, without properly determining if the consolidant is compatible with them. Some of the factors affecting the performances of consolidants are known, such as depth of penetration and moisture transfer through consolidated stone. However, insufficient consideration has been given to equally important factors such as their consolidating abilities and the compatibility of their thermal-dimensional properties with stone. Finally, the long-term performances of consolidated stone of historic structures are rarely documented.

These considerations point to the inadequacy of the present state of stone consolidation and conservation technology. For example, stone consolidants should be selected on the basis of an understanding of the deterioration processes of stone, of the factors affecting the performances of consolidants, and of the compatibility of consolidants with specific stones. Presently, such information is often not available. Further, standard test methods and performance criteria should be developed to form the basis for selecting promising consolidants. Documentation of the performances of stone consolidants should be an integral part of each preservation or restoration program. Documentation of unsuccessful consolidation work is just as important as documenting successful work in that it assists other stone conservators in rejecting ineffective materials and methods.

This review clearly indicates that a perfect stone consolidant has not been developed and that many of the consolidants can do more harm to stone than natural weathering processes. Therefore, the general use of stone consolidants is open to question. In fact, the Commonwealth War Graves Commission, which is responsible for over one million headstones in Europe, has concluded [2] that no type of consolidant should be applied to headstones. This commission has over 50 years experience with the chemical treatment of stone. There are cases, however, in which the use of stone consolidants can be beneficial. The work by Hempel and Moncrieff [115, 116, 131, 138, 139] has shown that decaying stone statues can be conserved by deep impregnation with certain stone consolidants. While statues and smaller objects can be removed to laboratories, surfaces thoroughly cleaned, freed from soluble salts, and all sides treated with a consolidant, such processes are not possible with massive stone structures. Risks involve in treating massive structures are, therefore, much greater. Consolidants might be used on structures of little historical or intrinsic value, and other cases where the benefits outweigh the involved risks [2]. For example, consolidants could be applied to deteriorated stone to delay to a future time the need to replace it with new stone. (This approach could be dangerous if the consolidation measure is later regarded as "permanent"). Any permanent consolidation effort involving important historic stone structures should be carefully planned and carried out to minimize the risks. This includes making certain that moisture and soluble salts cannot become entrapped behind the treated stone. In addition, the compatibility of a consolidant with a specific stone should be determined with separated or isolated test specimens rather than using a historic structure as an experiment.

Stone conservators should be cautioned in the indiscriminate use of newly developed materials which have shown promise in accelerated laboratory tests. While these tests are useful in determining what materials are unacceptable, good results are not unequivocal proof that a material will have a lasting beneficial effect. Further, at present, we are not able to predict the durability of consolidated stone for even 20 to 30 years. Yet, unproven consolidants are being used on stone structure which have lasted for hundreds of years and which with rational conservation programs will last for many more years.